This blog shares with you the knowledge about the operation of C language files, exchanges with each other and makes common progress

What is a file?

The code we wrote earlier uses "variables", or memory

Memory characteristics

Small capacity, speed block, data loss after power failure, relatively expensive

But memory can't meet all our usage scenarios. Sometimes we also want to have a larger storage capacity. It doesn't matter if the speed is a little slower. We hope that the data can be stored permanently. At this time, we use external memory, such as stored in the hard disk

External memory characteristics

Large capacity, slow speed, data still exists after power failure, which is cheaper than memory

  So how do we store data in external memory? The key is through documents  

Path to file

There are many files on our computer, so how does the operating system manage files? The key here is the path

eg.

E: \ embedded development learning \ development environment configuration 2021.docx

This is the path of the development environment configuration 2021.docx file on my computer

Note: the path of each file must be different on the same machine

File name suffix

File name suffixes are also called extensions

Some people may find that the files on their computer can't see the extension. This is because your computer hides the suffix. You can display the extension in the following ways

  So you can see your file extension

Extension function:

1. Describe the data format in this file
2. This file should be opened using that program

Document classification

  What we're talking about here is data files

How to operate files in C language?  

The standard library also provides a FILE structure to describe a FILE

Files are stored on disk. It is not easy to directly operate the disk, so the operating system is encapsulated. When you open a FILE, you actually create a variable (FILE structure variable) in memory, which is associated with the FILE on the disk. Therefore, reading / writing this variable is equivalent to reading and writing the FILE.

C language standard library provides a set of operation functions (header files are < stdio. H >)

Here we learn some of the main functions

• fopen
• fclose
• fwrite
• fread

fopen

Used to open files

(in fact, a FILE structure is created in memory)

Function declaration

FILE* fopen (const char* filename, const char* mode);

Return value:

FILE* :   field name pointer

If the file fails to be opened (the file does not exist or has no read-write permission), NULL is returned

Parameters:

filename: this parameter indicates a file path

mode: how to open the file

There are several basic ways to open

For text files:

1. "r"       read
2. "w"       Write       Clear the original file data and write from the beginning of the file
3. "a"       Add     Write the data to the end of the original file

Open for binary: "rb", "wb", "ab"

code

#include <stdio.h> 
int main() {
	FILE* fp = fopen("e:/test.txt", "r");
	printf("%p\n", fp);
	return 0;
}

Use this code to open the test.c file of disk d

Operation results:

  You can see that the print result is 0, which is because the opening failed and the return value is NULL (because there is no such file on e disk of my computer)  

We should judge the operation of opening files as follows:

#include <stdio.h> 
int main() {
	FILE* fp = fopen("e:/test.txt", "r");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	return 0;
}

Operation results:

  There is an error code (errno) in the standard library to indicate the failure reason

There will be a corresponding error code when there is a problem in the operation of the program

You can use the strerror function to translate error codes

Using perror this function can easily print error codes

Let me create a text.txt file on disk e to have a look

fclose

This function closes the file

Why close the file?

This is similar to memory leakage. If we do not release the dynamically developed memory, it will cause memory leakage

If fclose is not performed after fopen, file resource leakage may be triggered (the number of files that a program can open at the same time is also limited). If the file is opened all the time without closing, it cannot be opened again to a certain extent

Function declaration

int fclose (FILE* stream);

  Return value:

If the file is closed successfully, 0 is returned; otherwise, EOF (- 1) is returned

Parameters:

Stream: result returned by fopen

The following code:

#include <stdio.h> 
int main() {
	FILE* fp = fopen("e:/test.txt", "r");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	fclose(fp);
	return 0;
}

Operation results:  

 fread

read file

(it is actually copying the data in the disk to memory)

Function declaration

size_t fread (void* ptr, size_t size, size_t count, FILE* stream);

Return value:

Number of elements actually read successfully

Parameters:

ptr: to read a file is to copy the data from the disk to memory, and ptr points to this memory space

size: how big is each element

count: how many elements to copy

stream: pass in the result returned by fopen and find the specific location of the corresponding file on the disk

The following code:

#include <stdio.h> 
int main() {
	FILE* fp = fopen("e:/test.txt", "r");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	char buffer[1024] = { 0 };
	size_t count = fread(buffer, 1, 100, fp);
	printf("%d\n", count);
	printf("%s\n", buffer);
	fclose(fp);
	return 0;
}

  The data "hello" has been saved in the test.txt file

Operation results:

Open the file through this code, then read 100 elements from the file, one byte for each element, print the number of successfully read elements, and print the contents in the buffer

fwrite

Write file

(copy data in memory to disk)

Function declaration

size_t fwrite (const void* ptr, size_t size, size_t count, FILE* stream);

Return value:

size_t type, indicating the number of elements successfully written

Parameters:

ptr: copy the data in memory to disk. ptr points to this memory space

Size: the size of an element is size

Count: a total of count elements

stream:   Pass in the result returned by fopen and find the specific location of the corresponding file on the disk

Code demonstration:

int main() {
	FILE* fp = fopen("e:/test.txt", "w");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	char buffer[1024] = "success";
	size_t count = fwrite(buffer, 1, strlen(buffer), fp);
	printf("%d\n", count);
	return 0;
}

Operation results:

 

  At this point, you can see that success has been saved in the test.txt file

Then let's run the following program again:

#include <stdio.h>
int main() {
	FILE* fp = fopen("e:/test.txt", "w");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	return 0;
}

Open test.txt

  It was found that the file was emptied after the code that only opened the file was executed

be careful

If you don't want to empty the original data, be sure to use "w" carefully

fprintf

Write data to file (format write)

See the following code:

#include <stdio.h>
int main() {
	FILE* fp = fopen("e:/test.txt", "w");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	fprintf(fp, "ret = %d", 2);
	return 0;
}

Operation results:

 

  You can see that the data is written to the file

fscanf

Format read

Then follow the operation of fprintf above

Modify the code as follows:

int main() {
	FILE* fp = fopen("e:/test.txt", "r");
	if (fp == NULL) {
		perror("fopen");
		return 0;
	}
	printf("File opened successfully\n");
	int ret = 0;
	fscanf(fp, "ret = %d", &ret);
	printf("%d\n", ret);
	return 0;
}

Operation results:

Successfully read data from file  

sscanf

Parse the content from a string

The following code:

#include <stdio.h>
int main() {
	char ch[] = "ret = 2";
	int ret = 0;
	sscanf(ch, "ret = %d", &ret);
	printf("%d\n", ret);
	return 0;
}

Operation results:

The result was successfully read from this string  

 sprintf

Output a formatted result to a string

The following code:

#include <stdio.h>
int main() {
	char ch[50] = { 0 };
	sprintf(ch, "ret = %d", 2);
	printf("%s\n", ch);
	return 0;
}

Operation results:

  The formatted result was successfully output to the string

Application scenario

For conversion between strings and numbers (integers / floating point numbers)

1. Convert string to integer

#include <stdio.h>
int main() {
	char ch[] = "11";
	char ch2[] = "12";
	int num1 = 0;
	int num2 = 0;
	sscanf(ch, "%d", &num1);
	sscanf(ch2, "%d", &num2);
	int ret = num1 + num2;
	printf("%d\n", ret);
	return 0;
}

Here, the contents of ch and ch2 are converted into integers through sscanf and added

Operation results:

2. Convert integer to string

#include <stdio.h>
int main() {
	char ch[50] = { 0 };
	int ret = 10;
	sprintf(ch, "%d", ret);
	printf("%s\n", ch);
	return 0;
}

Operation results:

-----------------------------------------------------------------

-----------C language file operation completed---------

Welcome to pay attention!!!

Learn and communicate together!!!

Let's finish the programming!!!

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